The present invention relates to a method for dispersion compensation regulation in an optical transmission system, having dispersion compensation units, for transmitting high-bit-rate optical signals.
In the case of existing optical transmission systems, particularly optical wavelength division multiplexing (WDM) transmission systems, optical signals or WDM signals are transmitted at high bit rates (40 Gbit/s and more) via optical fiber link sections. During the transmission of optical signals, fiber dispersion caused by the different group delay times of the optical transmission fibers occurs, in particular, in addition to further nonlinear effects. The error-free transmission of optical signals via such an optical transmission system requires that this optical fiber dispersion be compensated with substantial accuracy. In this case, the tolerance range of an error compensation is directly proportional to the inverse square of the bit rate of the optical signal to be transmitted; that is to say, the higher the bit rate of the optical signal the more accurately the dispersion compensation must be.
This results in a very narrow tolerance range for the dispersion compensation regulation for the transmission of high-bit-rate optical signals with bit rates bigger than 50 Gbit/s. Such an accurate regulation of the dispersion compensation is associated with a high technical outlay, since the fiber dispersion of already laid optical fiber links or optical transmission fibers can be estimated only with low accuracy. For this purpose, for example, an optical test signal, which can be a random signal, is fed at the transmitting end into the optical transmission system and the bit error rate of the transmitted optical test signal is determined at the receiving end. Dispersion compensation is regulated by the minimization of the determined bit error rate of the transmitted optical signal.
Methods are known for this purpose in which the absolute-magnitude compensations of the dispersion compensation units are set in accordance with an estimated fiber dispersion, and the bit error rate (BER) of the transmitted optical WDM signal is subsequently determined. Known for the purpose of determining the bit error rate are a number of different methods that are used with commercially available measuring instruments to determine the bit error rate; for example, from Agilent. By varying the absolute-magnitude compensations of the dispersion compensation units, regulation is performed to a minimum bit error rate, and the optimum setting of the dispersion compensation units is thereby determined. However, the estimation of the absolute-magnitude compensations proves to be disadvantageous in the communication of high-bit-rate optical signals, since most estimated absolute-magnitude compensations lead to a bit error rate (BER) far removed from the minimum. In this case, it is virtually impossible to determine the bit error rate, since the forward error correction (FEC) of the optical receiving unit is no longer capable of correcting such high bit errors. Finding a suitable estimated value for the absolute-magnitude compensations is, therefore, extremely time-consuming and cost-intensive and does not always lead to an optimum dispersion compensation of the optical transmission links to be set up.
An object to which the present invention is directed lies in improving the dispersion compensation regulation carried out with the aid of a bit error rate minimization within an optical transmission system.